In recent years, a lot of research have been made on a next generation mobile communication system, and as a system for improving the frequency use efficiency of the system, a single frequency reuse cellular system has been proposed that each cell uses the same frequency band so that each cell can use the entire band allocated to the system.
In a downlink (communication from a base station device to a mobile station), an OFDMA (Orthogonal Frequency Division Multiple Access: orthogonal frequency division multiple access) system is the most available candidate. The OFDMA system is such a system that communication is performed by allocating radio resources configured in the time and frequency domains for a plurality of mobile terminal devices flexibly with the use of an OFDM signal performing communication by applying different modulation schemes to information data according to a reception state, such as 64 QAM (64-ary Quadrature Amplitude Modulation: 64-ary quadrature amplitude modulation) and BPSK (Binary Phase Shift Keying: binary phase shift keying).
In this case, since the OFDM signal is used, PAPR (Peak to Average Power Ratio: peak to average power ratio) may become very high, and the high peak power does not cause a major problem in downlink communication that relatively has an allowance for a transmission power amplification function, but causes a fatal problem in a mobile terminal in an uplink (communication from a mobile station to a base station device) that has no allowance for the transmission power amplification function. Thus, a communication system based on a single career system having low PAPR is desirable in the uplink.
However, when the single career system is used, it is impossible to perform control according to the reception state by the frequency axis like in the OFDMA system, and therefore, in order to improve the transmission efficiency, an SC-FDMA (Single Carrier-Frequency Division Multiple Access) system in which after recognizing the reception state of the entire system band, a single carrier signal of the time axis is converted into a frequency signal by a time frequency conversion such as discrete fourier transform (DFT: Discrete Fourier Transform) and the frequency signal is mapped to a part of the frequency band with a good reception state is employed as a transmission system of the uplink in a next generation cellular system called LTE (Long Term Evolution).
Further, in the SC-FDMA system, in order to control a signal by discrete frequencies, a part of a waveform at the rear part of a frame having a time longer than a maximum delay time of a delay wave in a radio propagation path with respect to a signal (hereinafter referred to as a frame in this specification) blocked for the time frequency conversion is added to the head of the frame as CP (Cyclic Prefix) and the CP is removed in a reception device, thus making it possible to circulate the effect of the delay wave of the propagation path in the reception frame. This makes it possible to represent an impulse response in the radio propagation path as an equivalently cyclic convolution, and it becomes possible to independently perform signal processing of a value of the discrete frequency signal obtained by the DFT.
Generally, equalization processing for compensating distortion by the radio propagation path is necessary in the SC-FDMA system, but in the case where control by the discrete frequency using CP is enabled as described above, a transmission equalization technology can be cited, wherein, after recognizing the impulse response of the radio propagation path in advance, the reverse characteristics of the frequency response are multiplied by the frequency signal for transmission. In the transmission equalization technology, where the distortion of a received signal has been completely compensated in the reception device, and a large amount of power is allocated to the discrete frequencies with low gain of the propagation path and a small amount of power is allocated to the discrete frequencies with high gain. Thus, in the uplink where a small-sized terminal performs transmission, the transmission efficiency of energy is lowered.
As a spectrum shaping technology for maximizing the reception energy in the reception device from a viewpoint of energy transmission, a water filling principle is widely known in the field of information theory, etc. FIG. 8 shows a concept of the water filling principle.
First, as shown in the same figure (a), if a received signal to noise power ratio is obtained when receiving with the same power for all the frequencies, it is able to be confirmed that the higher the frequency of the received signal to noise power ratio is, the higher the transmission efficiency of energy becomes, and the lower the frequency is, the lower the transmission efficiency of energy becomes.
Next, as shown in the same figure (b), after converting into a received noise to signal power ratio which is an inverse of the received signal to noise power ratio, the straight line P in parallel to the abscissa axis is set to satisfy the following condition with respect to the graph. An area of a part, which is positioned below the straight line P and surrounded by the noise signal power ratio and the straight line P, (a part indicated by the diagonal line of the same figure (b)) is a transmission power Q. At this time, since a difference between the straight line and the noise signal power ratio of each frequency is the transmission power Q of each frequency, it is possible to determine the transmission power to be allocated to each frequency as shown in the same figure (c). This is called a water filling principle since the transmission power distribution is optimum when an amount of water corresponding to the transmission power is poured from the top to the noise signal power ratio of the same figure (b) and a depth of gathered water is set as the transmission power Q.
By applying the water filling principle, clipping processing is applied that a larger amount of energy is allocated to the frequency with high gain of the propagation path and a smaller amount of energy is allocated to the frequency with low gain, further no power is allocated to the frequency with significantly low gain (a part R to which no transmission power is allocated of FIG. 8(b)). In this case, while it is possible to maximize the reception energy from a viewpoint of energy, the number of paths (the number of taps) of the impulse response of the propagation path increases and inter-code interference, which is interference of signals in a frame, is highlighted, thus posing a problem that the maximized reception energy can not be utilized as a result.
Against this, in non-linear iterative equalization having an excellent interference prevention function as represented by turbo equalization, on the assumption of error correction coding, reliability is improved with equalization for improving reliability of a transmission bit by removing distortion by the propagation path and decoding for enhancing reliability of the transmission bit by error correction processing, and the improved reliability information is mutually transmitted between an equalizer and a decoder as prior information, which is repeated to realize complete equalization also for a signal to which spectrum shaping has been applied by the water filling principle.
Here, in the spectrum shaping technology using the turbo equalization technology, when iterative processing converges, the turbo equalization enables to completely prevent inter-code interference by the spectrum shaping and the radio propagation path and to combine delay wave components spread on the time axis, thus making it possible to utilize the reception energy maximized by the water filling principle effectively (for example, Non-Patent Literature 2). Here, the convergent state of the iterative processing refers to the state where the enhancement of reliability by equalization and decoding are not stopped halfway and information of the transmission bit is able to be recognized completely.
When the spectrum shaping technology is used on the uplink by a plurality of transmission devices, spectrum shaping by the water filling principle is performed as primary spectrum shaping. At this time, when transmission is performed using the same time and the same frequency band, a part of spectrum overlaps. Accordingly, clipping is performed assuming that a part of frequency (spectrum) for which no clipping is performed by the primary spectrum shaping is used for transmission in other transmission devices, and power is redistributed to the frequency used for transmission. This makes it possible to multiplex signals of transmission devices without reducing a transmission bit rate (for example, Non-Patent Literature 3).
FIG. 9 shows a concept of spectrum shaping by clipping when two transmission devices communicate with a base station. The same figures (a) and (b) show an example of the concept of primary spectrum shaping to maximize reception energy and secondary spectrum shaping to multiplex signals from a plurality of transmission devices, respectively.
First, in the same figure (a), each of the transmission devices performs spectrum shaping based on the water filling principle capable of maximizing the reception energy (overlapping spectrum B101). Then, in the same figure (b), by clipping a part of spectrum as the secondary spectrum shaping (spectrum that is subjected to the clipping C101), it is possible to multiplex signals between the transmission devices so as not to be overlapped. The clipping in the secondary spectrum shaping is performed for the spectrum notified from the base station device. The base station selects the frequency used for transmission successively, the frequency having the highest gain of the propagation path alternately from among discrete frequencies overlapped between transmission devices (so as to avoid overlapping of spectrum among users). After securing a predetermined band, the base station performs scheduling by means of clipping the remaining frequencies and notifies the scheduling result to each of the transmission devices. In this way, when signals of the transmission devices are multiplexed by frequency, the frequency having a good propagation state for a certain user can not be used for transmission, and therefore it can be considered such that spectrum shaping is further performed for the optimum water filling principle to realize both quasi-optimum spectrum shaping and frequency-multiplex of signals.
Next, as to the convergent state when separation and detection of signals by the non-linear iterative equalization are complete, it is known that there is also a non-convergence case but behavior thereof can be performed visually by extrinsic information transfer (EXIT: EXtrinsic Information Transfer) analysis. FIG. 10 shows a block diagram of an analysis model, FIG. 11(a) shows an example of a convergent state, and FIG. 11(b) shows an example of anon-convergent state. First, in FIG. 10, the analysis model is configured by an equalizer 1101 and a decoder 1102, serving as a model in which improved reliability is transmitted mutually. At this time, in the equalizer 1101, equalization processing needs a received signal, propagation path characteristics, an average received signal to noise power ratio, and reliability obtained by the decoder 1102. On the other hand, in the decoder 1102, reliability of the transmission bit obtained by the equalizer 1101 is input and error correction processing is applied so that improved reliability is output.
The EXIT analysis is illustrated with input and output characteristics of mutual information (MI: Mutual Information) in order to represent mutually improved reliability of the transmission bit quantitatively. First, mutually transmitted in the turbo equalization processing technology is a log likelihood ratio (LLR: Log Likelihood Ratio) of the transmission bit that natural logarithm (logarithm whose base is e (Napier's constant)) is obtained for a ratio of the probability that the transmission bit is 1 to the probability that the transmission bit is 0. Here, MI related to the transmission bit obtained from LLR is constrained to 0 to 1, and 0 shows that no information related to the transmission bit is obtained at all and 1 shows that information related to the transmission bit is completely obtained.
This is shown in FIGS. 11(a) and (b) as input and output characteristics of the equalizer 1101 and the decoder 1102. In these figures, the abscissa axis shows the input MI of the equalizer and the ordinate axis shows the output MI of the equalizer. This also means that since the output MI of the equalizer is the input MI of the decoder in the turbo equalization, the axes of the input and output MI characteristics of the decoder and the input and output MI characteristics of the equalizer are reversed in the same graph.
First, FIG. 11(a) illustrates the convergent state of iterative processing, where L1101 shows the input and output characteristics of the equalizer and L1102 shows the input and output characteristics of the decoder. In the same figure, starting from an origin, since there is no prior information at first, the MI is obtained by the equalization processing as shown by A1101. Next, since the MI obtained by the equalization processing is the input MI of the decoder, advance is made horizontally as shown by A1102 so that improvement is obtained by error correction. Similarly, when tracks are drawn in the order of A1103 and A1104, respective input and output characteristics will be not crossed, thus 1 is eventually obtained in the output MI of the decoder, and enabling to recognize the transmission bit completely. On the other hand, in FIG. 11(b), respective input and output characteristics are crossed, and when drawing tracks, no improvement is made at the intersection point. This state is called stack in which a detection error is not eliminated even after repetitions based on the turbo principle and the iterative processing is brought into the non-convergent state.
This is because of the propagation path which is changing momentarily. Aiming to deal therewith, an adaptive coding modulation scheme has been also proposed, wherein, from among the combinations of a modulation scheme resulting in the convergent state and an encoding ratio of error correction coding, a combination of the modulation scheme and the encoding ratio allowing to transmit most information bits is adaptively selected (for example, Non-Patent Literature 4).